The present invention relates to a method for producing a large-mass ohmic resistor for protecting electronic assemblies from surges and to an electronic assembly having means for protection from surge voltage or surge current pulses.
In electronic assemblies, protection against high-energy overvoltage spikes is necessary to avoid destruction of the components.
IEC1000-4-5 for testing immunity to surge voltages contains information on the limiting values and waveforms for corresponding tests. Usual test levels are from 0.5 to 4 kV or higher, depending on the required overvoltage categoxy of the electronic assemblies. In the unloaded condition, the surge voltage pulse (surge load) has a double exponential waveform having a front time of 1.2 μs and a time to half-value of 50 μs. In this context, the front time is defined as the rise time of a surge (voltage) pulse from 10% to 90% of its amplitude peak value, whereas the time to half-value is defined as the time of the surge (voltage) pulse from the maximum (100%) of the amplitude peak value to 50% thereof. In line-to-line testing, an impcdancc of 2Ω is specified as the internal impedanec of the surge generator.
Generally, it is not possible to design the components for such an overvoltage. Therefore, it is usual to limit the voltage at the electronic assembly using voltage-limiting components, for example, by varistors or suppressor diodes, which are connected in parallel to the component (FIG. 1). Due to the low internal impedance of the test generator, very large currents flow via the voltage-limiting protection element. In the process, a very large pulse energy must be absorbed. To achieve a high protection level, it is therefore necessary to use a relatively large, voluminous varistor having a corresponding absorbing capacity. Moreover, the clamping voltage, i.e., the voltage limited by the protection element, increases as a function of the non-linearity component of the protection element and of the current pulse level.
As an example, in an application using a 275V varistor at a pulse voltage of 4 kV with a phase angle of 90° to the line voltage, a maximum pulse current of about 1700 A occurs, resulting in a maximum clamping voltage of 900V. Since IEC1000-4-5 requires a sequence of twenty pulses spaced sixty seconds apart, a varistor having a minimum diameter of 14 mm is required for this load. In devices having a lower power requirement, it is therefore common practice to increase the input impedance of the electronic assembly by means of a series resistor (FIG. 2) to reduce the pulse currents. In the example mentioned above, given an input impedance of about 50Ω, the pulse current is limited to a maximum value of 70 A. On one hand, this results in a lower clamping voltage of 750V maximum and, on the other hand, the varistor can be reduced to a disk diameter of 5 mm here. Because of this, it is also possible to use varistors in SMD technology (surface mounted device), which are currently only manufactured up to a maximum pulse load capacity of 1200 A (single pulse).
In this context, it is a disadvantage that a resistor having a very high pulse immunity has to be used for the series resistor. In the example mentioned above, the resistor must withstand a pulse power of about 240 kW. However, modern resistors in film technology are unsuitable for such a pulse load. Carbon composite resistors, which have excellent pulse immunity, are nowadays hardly produced anymore. Therefore, only wire-wound resistors are suitable as a series resistor. However, for the example mentioned above, a size having a power rating of 4 Watts minimum is required, depending on the type of resistor. However, these resistors have a very voluminous design and are available as through-hole mounted devices only. Moreover, the expenses are higher here than when using a larger varistor for “hard clamping”, that is, for a suppressor circuit without additional series resistor.
Using SMD wire-wound resistors, which are currently manufactured with a maximum power rating of 2.5 W, a surge load of 2 kV maximum can be achieved.
Furthermore, the general use of printed series resistors for LEDs is described in “National Technical Report”—“Thin panel switch”, August 1996, Matsushita Electric Industrial Co, Japan, Bd. 42, Nr. 4, pages 50–56.